This invention relates in general to image enhancement in digital image processing and in particular to the tone scale rendering of selected regions of interest to improve the visibility of radiographic image details.
The growing availability of information technology is having an important influence on medical imaging practice. Picture archiving and communications systems allow for digital acquisition, image processing and display for a wide variety of medical imaging modalities in a fully digital, filmless environment. The many advantages promised by digital medical imaging include increased productivity for the health care professionals who need to utilize images as part of patient care. Therefore, it is important to rapidly extract diagnostically useful information from medical images. This requires that minimal user interaction be required. When the user interaction is required with the image display, it must be rapid, intuitive and as automated as possible.
One source of medical images is the detection of x-rays projected through a region of interest of a patient so as to measure the x-ray transmittance with an imaging detector of finite area. Such images are described as projection radiographic images. Digital radiographic images often capture a wide range of x-ray exposures that must be rendered on a single image. A tone scale curve is used to selectively map code values representing a standardized input (for example, the logarithm of x-ray exposure) to standardized output (for example, optical density). A well-chosen tone scale curve provides an acceptable compromise between contrast and latitude for the overall image. However, there are often regions of the image that are rendered too dark or too light for optimal interpretation. There is a need to automatically improve the contrast and brightness of the overly dark or light areas without affecting the tone scale of the remainder of the digital image.
When printed films and a light-box are used for the display of radiographs, an ancillary small intense source of light know as a xe2x80x9chot lightxe2x80x9d or xe2x80x9cbright lightxe2x80x9d is often used to better visualize dark areas of the film image. By projecting a more intense light source through the dark areas of the film, the amount of light reaching the viewer""s eye is increased. This is generally found to aid visualization of structures and details in the otherwise too dark areas. The utility of a xe2x80x9chot lightxe2x80x9d is limited for two reasons. The area illuminated by the xe2x80x9cbright lightxe2x80x9d is necessarily limited so that only the dark areas are illuminated. Therefore, the context of the unlighted areas of the image is lost to the viewer. There is also no contrast enhancement in the area of interest; this is desirable because the contrast of film images generally decreases at high densities. Additionally, a xe2x80x9cbright lightxe2x80x9d is of no help for too light areas of the film and no analogous method exists to improve visualization in these areas.
When radiographic images are viewed with electronic displays, adjustment of window width and level is a standard practice used to improve the rendering of a region-of-interest. This is most often done manually by means of a mouse or track-ball and requires substantial time and effort to obtain a desired result. Window width and level adjustment requires a skilled operator to be accomplished successfully. Further, adjusting window width and level for a region of interest often has the effect of making the remaining areas of the image sub-optimally rendered. In addition to this loss of image quality, adjusting window width and level can also cause a loss of context for the region of interest.
U.S. Pat. No. 5,542,003, issued Jul. 30, 1996, inventor M. Wofford, describes a method to automatically adjust the window and level parameters for an image based on statistics of a selected region of interest. In this patent, the window and level parameters are applied to the entire image. This causes a sub-optimal rendering outside the selected region of interest and eliminates the context of the surrounding regions. Also, the algorithm does not suggest applying the contrast adjustment prior to performing the tone scale mapping. Applying the contrast adjustment after tone scale processing introduces unnecessary quantization to the enhanced region of interest.
The commercial image editing and manipulation application Adobe Photoshop (Adobe Systems Incorporated, San Jose, Calif.) provides the following technique. A region of interest in an image may be selected and an operation called xe2x80x9cAuto Levelsxe2x80x9d performed. The selected region of interest is then processed so that the resulting histogram is linearly stretched across the available dynamic range. This is analogous to automatically selecting window/level parameters for a region of interest while leaving the remaining image unmodified. The processing operation is not performed before the tone scale mapping is applied. This results in a reduction of image quality due to quantization in the toe or shoulder regions of the tone scale. Additionally, expanding the code values of the region of interest across the entire available dynamic range can yield excessive contrast in the region of interest, creating an unnatural appearance.
U.S. Pat. No. 6,017,309, issued Jan. 25, 2000, inventors Washburn et al., disclosed a method for automatically adjusting the color map of color flow ultrasound data in a selected region of interest. As disclosed, the color flow estimates are overlaid on top of the gray scale display image, then the color map of the color flow data is automatically adjusted. Therefore, two different data sets are being viewed simultaneously.
U.S. Pat. No. 6,047,042, issued Apr. 4, 2000, inventors Khutoryansky et al., discloses an exposure control system for radiographic and fluoroscopic imaging applications based on sensor elements in a selected region of interest. This technique modifies the acquisition parameters for an image and affects the entire image globally.
According to the present invention, the drawbacks described are eliminated.
According to a feature of the present invention, there is provided an automated computationally efficient method for enhancing user selected regions of interest that may be too light or too dark to be well visulaized.
According to another feature of the present invention, there is provided a method for automatically modifying the rendering of an image based on an analysis of pixel values without a selected region of interest comprising:
providing a digital input image of digital pixel values and tone scale look-up table;
creating a default rendered image by applying said tone scale lookup table to said input image;
displaying said default rendered image;
selecting a region of interest from said input image computing the histogram of the pixel values within said region of interest; and
Creating a bright light by remapping the pixel values within the said region of interest based on an analysis of said histogram and said tone scale look-up table and overlaying said bright light image on said default rendered image.
The invention has the following advantages.
1. The context of the original image is maintained by operating only on the selected region of interest, the remainder of the image is unchanged.
2. The image quality of the selected region of interest is improved over traditional window and level adjustment because the contrast of the selected region of interest is increased without incurring quantization due to the toe and shoulder of the tone scale look-up table application.